Laser pulse shaping

ABSTRACT

A system for temporal pulse shaping of laser radiation beams particularly for use as photoexcitation radiation in isotope separation. Each pulse of laser radiation is temporally segmented into two or more sections. The sections may be superimposed after a delay in the leading one to augment the pulse intensity, or the segmented pulses may be separately used as segments in a desired application, or the pulse rise time may be decreased by segmenting the pulse so as to permit rejection of the more slowly rising leading edge.

FIELD OF THE INVENTION

The present invention relates to a system for temporal segmenting ofradiation pulses and spatially separating the different segments.

BACKGROUND OF THE INVENTION

In applications for pulsed laser beams such as isotope separation and inparticular uranium enrichment, examples of which are shown in U.S. Pat.Nos. 3,772,519; 3,939,354; 3,924,937; and 3,944,947, pulsed radiation isemployed to produce isotopically selective photoexcitation and/orionization of a vapor environment of the particles of plural isotopetypes, one or more types of which are to be separated. In providing thelaser radiation pulses for this application, it is common to generate abeam of a precisely defined spectral purity to insure selectiveexcitation and to provide amplification in one or more stages ofamplifiers to a useful energy level. Laser radiation pulses generated insuch a manner typically have a conventional bell-shaped pulsecharacteristic. The slow rise and fall shape is typically too long induration for the period of time during which useful photoexcitation maytake place, typically approximately one microsecond. The excessivelyslow rise time necessitates wasting or undesirable preapplication of theleading edge of the radiation pulse until a sufficient intensity forefficient photoexcitation is reached. This can result in an undesirableloss of radiation energy and efficiency.

BRIEF SUMMARY OF THE INVENTION

In accordance with the teaching of the present invention a system fortemporal pulse splitting is disclosed for particular application toisotope separation by photoexcitation. The temporal splitting may beemployed to increase the leading edge rise time of the pulses of laserradiation, to shorten the pulse duration and thereby provide two or morepulses from each single amplified pulse of radiation with a timeduration appropriate for photoexcitation, or permit superposition afterappropriate delays of the segmented pulse sections to result in anincrease in laser pulse intensity.

In particular implementation, a beam of pulsed laser radiation isdirected through an element which is operative to change acharacteristic of the radiation in response to an electrical signalwhich is provided to switch the characteristic at some intermediatepoint during each pulse of laser radiation. The pulse thus segmented isdirected to a further element which is operative to provide spatialseparation of the radiation pulses based upon whether the characteristichas or has not been changed. In this manner, separate paths of radiationfor each segment desired may be achieved. It is possible to segment thepulse directly subsequent to the slow rising edge thereby to produce asubstantially shorter pulse with a more rapid rising edge forapplication to the environment which is to be selectively photoexcited.The radiation may also be segmented into two or more generally equalpulses of radiation which may then be separately directed onto two ormore paths through the plural isotope environment to produce separateregions of selective photoexcitation.

In a particular embodiment, the laser radiation pulses are applied to aPockels cell or Kerr cell which is electrically driven to rotatepolarization of applied radiation 90°. The selectively rotated radiationis applied to a polarizer, such as a Glan Thompson crystal or any otherlow loss polarizer, in which the radiation of unshifted polarizationpasses through one channel of the crystal, while a 90° shift in thelaser radiation polarization causes it to exit from the crystal on adifferent angle.

For segmenting the laser pulses into greater than two sections, a matrixof polarization rotators and polarization sensitive crystals may beemployed so that three or more beams of pulsed radiation may be obtainedfrom a single input pulse.

The segmented pulses are also time delayed to produce time coincidenceand are then superimposed for increased intensity.

BRIEF DESCRIPTION OF THE DRAWING

These and other features of the present invention are more fully setforth below in the exemplary and nonlimiting detailed description andaccompanying drawing of which:

FIG. 1 is a schematic diagram of a system for providing temporal pulsesplitting of pulsed radiation;

FIG. 1a shows a modification to the FIG. 1 diagram;

FIG. 2 is a waveform diagram illustrative of a segmenting functionprovided by the apparatus of FIG. 1;

FIG. 3 is a system diagram of the use of the present invention in a setof lasers employed for isotopically selective photoexcitation;

FIG. 3a shows a modification to the FIG. 3 diagram;

FIG. 3b shows a waveform diagram useful in understanding themodification of FIG. 3a;

FIG. 4 is a schematic diagram of apparatus utilizing the radiationgenerated in FIG. 3 for selective excitation and ionization; and

FIG. 5 is a sectional internal view of the apparatus of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

The present invention contemplates a system for segmenting pulses ofradiation, typically laser radiation, to reduce the time duration ofindividual segments, sharpen the rise time, and/or permit superpositionof the segments after time shifting into coincidence.

The purpose of segmenting such pulses of radiation has its origin in theapplication of laser isotope separation, typically uranium enrichment,as presented in the above-identified U.S. patents. Pulses of laserradiation generated for the purpose of pulsed photoexcitation of a vaporof plural isotope types is typically too long in duration due at leastin part to the length of laser amplifier excitation radiation providedby conventional flashlamp excitation sources used for high power dyelaser amplification. The result is a laser pulse duration, approximatingtwo microseconds.

The response of the uranium vapor environment to excitation radiation isto excite a number of particles in the vapor to an excited level. In theapplication to isotope separation the excitation is made isotopicallyselective by properly tuning and band limiting the frequencies ofexcitation radiation.

In typical application, plural frequencies of radiation or at leastplural photon interactions will be necessary to photoionize particles inthe process of isotope separation outlined in the above-referencedpatents and as a result excited atoms will reside in the excited stateuntil they either spontaneously decay or are further excited or ionized.It is clearly preferable that excited particles experience the latter,i.e. further excitation or ionization rather than spontaneous decaywhich would mean that they were lost from the system and that the energyexpended in exciting them has been wasted. It is accordingly importantthat radiation employed for further excitation of already excitedparticles be timed precisely so as to occur, with full intensity beforea significant number of excited particles can decay. This necessitatesamong other things that the pulses of radiation employed for furtherexcitation have a sharp rise time in contrast to the relatively slow,bell-shaped leading edge typical of pulses of laser radiation as isshown, for example, in a curve 10 in FIG. 2.

In addition, it has been found that within an interval of timesignificantly shorter than the typical pulse duration from dye laseramplifiers, the process of photoexcitation and photoionization in theuranium vapor, for preferred transitions, reaches most of the atomsavailable for photoexcitation and photoionization. This indicates that aportion of the laser radiation in each pulse is wasted.

The use of laser pulse segmenting in accordance with the presentinvention permits splitting the long duration pulses into shortersegments which may then be applied to separate regions of the uraniumvapor for photoexcitation and photoionization thereby permittingefficient use of the energy employed in generating the laser radiationwithout resort to such tactics as flashlamp quenching.

The method and apparatus according to the present invention may now bebest understood by reference to the drawings and in particular to FIGS.1 and 2 which should be considered in conjunction with each other. InFIG. 1, a laser 12 is shown which is typically a tunable dye laseramplifier having a flashlamp excitation source 14 that is triggered by atimer 16 to excite the dye medium of laser 12 to a lasing conditionresulting in the output of a beam 18 from the amplifier 12 of highlyamplified, pulsed laser radiation. The beam 18 typically includes aseries of radiation pulses of the general, time versus intensity,waveform illustrated in waveform 10 of FIG. 2.

The timer 16 is implemented to provide in addition to a time pulse 20for activation of the flashlamp 14 as illustrated in FIG. 2, a triggersignal initiated approximately with the peak of the waveform 10 and inthe form of a rectangular pulse 22 which is applied to an element 24 inFIG. 1 capable of changing a characteristic of the radiation in the beam18 in response to the presence of the pulse 22. Typically, the element24 is a Pockels or Kerr cell in which the polarization of the radiationin the beam 18 is rotated, typically by 90°, such that the beam 26passing from the cell 24 is polarized the same as the beam 18 in theabsence of the pulse 22 but is polarized orthogonally in the presence ofthe pulse 22. For this purpose, it is preferable to align the Pockelscell with respect to the polarization normally present in the beam 18 sothat the rotation is optimally effective to provide a 90° polarizationshift. The beam 26 is further applied to a polarization sensitivecrystal 28, such as a Glan Thompson crystal, which is operative totransmit the incident radiation in beam 26 as an output beam 30 in afirst polarization and to deflect it as an output beam 32 in anorthogonal polarization. The result is that the paths 30 and 32 willeach receive a temporally distinct section of the segmented pulsewaveform 10 as illustrated by waveforms 34 and 36 in FIG. 2. It shouldadditionally be noted that the waveforms 34 and 36 have a durationsubstantially half that of the duration, at useful intensity levels, ofthe waveform 10. As such, the paths 30 and 32 may be applied forseparate utilization in isotopically selective photoexcitation andionization, making more efficient use of the total radiation energyunder the envelope of the waveform 10. This feature and apparatus foraccomplishing it is more fully illustrated below.

The radiation on the paths 30 and 32 may be superimposed by applying thewaveform pulse 34 on path 30 through a delay line 42 which delays it byan amount approximately equal to one-half of the duration of the pulse10. The delay line 42 may be a set of mirrors providing a sufficientpath length to accomplish the slightly less than one microsecond delaydesired. The delayed pulse may then be applied to a polarizing crystal44 in a polarity which transmits it through to an output path 45 incombination with radiation on the path 32 which, for this purpose, isalso applied to the polarizing crystal 44 along an input axis to whichit is responsive at the polarization on path 32 to redirect it onto thepath 45. In this manner, superimposed radiation having in general formthe wave shape of envelope 43 in FIG. 2 is provided. Not only is therean increase in peak intensity, but a more rapid rise time and a moreeven distribution of the peak intensity radiation. Also, the pulseduration has been shortened to coincide with the time of most effectivephotoexcitation or ionization.

It may alternatively be desired that the timer 16 generate a set oftiming signals as illustrated by the pulse train 40 in FIG. 2. As shownthere, the initial pulse 20 triggers the flashlamp and a subsequentrectangular wave pulse 46 is provided to span the approximate center,peak intensity portions of the waveform in envelope 10. The result willbe that the output beam 30 will correspond to the pulse of waveform 10having a center segment removed from it as illustrated in waveform 48while the output beam path 32 will contain the center segment asillustrated in waveform 50.

It is preferable then that the radiation on the output beam path 30 bediverted through a further Pockels or Kerr cell 52 which responds to thetimer 16 and in particular to a rectangular waveform 54 illustrated inpulse train 40 and following directly subsequent to the rectangularpulse 46 so as to rotate the polarization in the second section of thetwo segment waveform 48. In this manner, an output path 56 from thePockels or Kerr cell 52 will contain the pulse segments of waveform 48in differing polarizations. A polarizing crystal 58 placed in the path56 will then generate two separate output beams 60 and 62 correspondingto the waveforms 64 and 66 illustrated in FIG. 2. In this manner, theoutput beams 32, 60 and 62 will each contain different thirds of theoriginal pulse of waveform 10. These separate paths may also be appliedto isotope separation apparatus to excite separate regions of vapor.

It may additionally be possible to unequally segment the envelope 10 inaccordance with the timing diagram of pulse train 70 such that veryshortly subsequent to the pulse 20, initiating flashlamp discharge, arectangular pulse 72 is generated which continues for the duration ofthe radiation pulse 10 producing, in the output beam 32, a radiationpulse of the form illustrated in waveform 74 of FIG. 2. As shown there,the pulse has been truncated at the rising edge to provide a rapid risetime without disturbing the remainder of the pulse. The rapid rise timeis particularly useful in situations where it is desired to producemultiple excitation of particles. In this case the radiation for thefirst excitation step can be provided with a sharp rising edge anddelayed in application until the radiation for subsequent excitationsteps has reached a peak or useful intensity level.

The Pockels cell may additionally be driven by waveforms of complexshape in order to produce certain laser pulse shapes such as a squarewave. FIG. 1a illustrates such a system in which a waveform generator 75is provided to generate a voltage drive to Pockels cell 24 ofpredetermined form such as waveform 77 which will vary the angularpolarization shift to vary the intensity in beam 30 typically to asquare wave.

In the application of the present invention to isotope separation and inparticular to uranium enrichment, as is more fully treated in theabove-referenced U.S. patents, it is typical that several colors oflaser radiation be generated and corresponding laser amplifiers such asthe amplifiers 80, 82, 84 and 86 illustrated in FIG. 3, each with theirrespective flashlamp excitation sources 88, 90, 92, and 94 be providedfor the amplifiers 80-86. The flashlamps 88-94 are typically controlledby a timer 96. Normally, the timer 96 provides coincident excitation ofthe flashlamps 88-94 though it is contemplated that slightly staggeredexcitation, in the sequence in which the lasers are to be utilized toprovide photoexcitation, may be employed.

The output of the laser amplifiers 80-86 is applied through respectivecells 98, 100, 102, and 104 which are activated by the timer 96 througha short delay circuit 106 and driver amplifiers 108. The delay 106 ispreferably employed so as to locate the rectangular pulse of the formshown in pulse 22 in FIG. 2 for switching the cells the appropriate timeinterval subsequent to the activating pulse for the flashlamps such aspulse 20. The outputs of the Pockels cells 98-104 are applied torespective polarizing crystals 110, 112, 114, and 116. The action of thepolarizing crystals 110-116 on the radiation from the cells 98-104results in the generation of separate output paths for each polarizedsegment of the radiation from the cells 98-104 onto respective sets ofoutput paths 118, 120, 122, and 124 for one polarization, and paths 126,128, 130, and 132 for the other polarization. The radiation on paths118-124 is then applied to a multi-frequency radiation beam combiner 134while the radiation paths 126-132 are applied to a multi-frequencyradiation combiner 136. The multi-frequency radiation combiners 134 and136 may be of several forms such as dichroic mirrors arranged in amatrix to combine the several frequencies. The output of the combiners134 and 136 are respective single beam paths 138 and 140 which may bethen applied to separate channels or regions of the isotope separationapparatus.

Apparatus for this purpose is further illustrated by reference to FIGS.4 and 5 where, as shown in FIG. 4, a chamber 150 is provided as morefully described in the above-identified U.S. Pat. No. 3,939,354. Thechamber 150 is evacuated to a low pressure of a fraction of a millitorrand has a vapor source 152, typically for uranium, which directs a vaporflow upwardly into a set of ion separator plates 154 through whichradiation in beams 156, resulting from the juxtaposition of the beams138 and 140 from the combiners 134 and 136 is applied after passingthrough a window 158 on an extension pipe 160. Radiation after usewithin the separator 154 may exit through a further pipe 162 and window164 for further use. Surrounding the chamber 150 are a series ofmagnetic field coils 168 which are energized by current from a currentsource 170 to provide a magnetic field within the chamber 150 and inparticular in the region of the vapor source 152 and ion separator 154.The magnetic field is applied axially parallel to the laser beams 156.

The interior of the chamber 150 is more fully illustrated in FIG. 5showing a sectional view of a portion of the chamber in FIG. 4. Asillustrated there, the magnetic field 172 created by the coils 168 isoperative to provide a magnetic field for deflection of an electron beam174 for application to a reservoir 176 of uranium to be evaporated anddirected into the region of the ion separator plates 154. The ionseparator plates 154 are shown to include a series of plates 178, 180,182 and 184 defining chambers 186, 188, and 190. Within the chambers186-190 further, shorter plates 192, 194 and 196 are providedsub-dividing each of the chambers into right- and left-hand sections.

The radiation in the beams 138 and 140 are typically applied inseparate, adjacent or symmetric segments of the chambers such asillustrated by the regions 198 and 200.

In this manner, the radiation from a single pulse has been divided intosegments which can be used to excite adjacent regions of the vaporenvironment within the chamber. Mirrors may be employed to reflect thesebeams through the regions either side of the plates 192 and 196. Threeregions may be excited from a single pulse by segmenting into threesections as illustrated above.

The regions illuminated by the photoexcitation and ionization radiationwill be populated by selectively ionized particles which may then becollected by pulsed application of a voltage pulse between the plates178-184 and 192-196 from a voltage pulse source 202 connected to thoserespective plates and activated by the timer 96 just subsequent to theillumination in each region. For this purpose, the pulse source 202 mayrequire several separate sources of individual pulses which are timesequenced in accordance with the timing of the radiation in the separatesegments. It may accordingly be desirable to direct the radiation ofdifferent time of occurrence into different chambers 186-190. Timesequencing of the pulses may, however, be avoided by the use of delaylines in the path 140, such as the delay line 42 illustrated above withrespect to FIG. 1, to provide time coincidence of all pulses.

The sequencing of the pulses of different colors may be scheduled suchthat the first excitation step radiation is of sharp rise time anddelayed to come on only after radiation for the other steps (which willnot be effective until the first step is taken) has achieved a usefulintensity. A system for this purpose is shown in FIG. 3a where a delay210 is placed in the beam 118 to delay its application an interval untilthe other beams reach full intensity and saturate the uranium vapor toprovide delayed pulse 212 shown in FIG. 3b. Pulse 212 is delayed aninterval t₁ from the beginning of pulses 214 and 216 for the othercolors. Where the beam 118 also has a slow rise time, it is desired toseparate the leading edge by a Pockels or Kerr cell technique as shownabove.

It is noted that the segmenting and superimposing techniques disclosedabove may further be practiced in various combinations of the disclosedstructure.

The above-described preferred embodiment is intended to be exemplaryonly, the actual scope of the invention being defined only in accordancewith the following claims.

What is claimed is:
 1. A system for pulse shaping pulsed radiationemployed in photoexcitation comprising:means defining an environment ofparticles; a source of pulsed radiation; means responsive to the pulsedradiation from said source for segmenting each pulse of radiation intotemporally distinct segments; means for separating the segments ontoseparate paths; and means for applying the pulse segments on at leastone of the different paths for producing photoexcitation of particles ofsaid environment.
 2. The system of claim 1 wherein said segmenting meansincludes means for dividing each pulse into approximately equal halves.3. The system of claim 2 wherein said pulsed radiation source producespulses of at least 2 microseconds duration.
 4. The system of claim 1wherein said photoexcitation producing means further includes means forproducing isotope separation.
 5. The system of claim 4 wherein saidisotope separation means includes plural elongate channels and havingthe segments on different paths applied to distinct channels thereof. 6.The system of claim 1 wherein said segmenting means includes means forproducing at least three segments which appear on three separate paths.7. The system of claim 1 wherein said segmenting means includes meansfor pulse shaping at least one of said segments.
 8. The system of claim1 wherein said segmenting means includes means for segmenting each pulseto provide an enhanced rise time in at least one segment and in whichsaid one segment is applied to the path for producing photoexcitation.9. The system of claim 1 wherein:said segmenting means includes:meansresponsive to each pulse of radiation for producing polarizationrotation thereof in response to a signal; means for generating saidsignal during each pulse; and wherein said means for separating includesmeans for separating onto said paths the radiation in each pulse inaccordance with the polarization thereof.
 10. The system of claim 9wherein said rotation producing means includes a Pockels cell.
 11. Thesystem of claim 9 wherein said rotation producing means includes a Kerrcell.
 12. A system for pulse shaping pulsed radiation employed inphotoexcitation comprising:a source of pulsed radiation; meansresponsive to the pulsed radiation from said source for segmenting eachpulse of radiation into temporally distinct segments; means forseparating the segments onto separate paths; and means responsive to thepulse segments on at least one of the different paths for producingphotoexcitation; said segmenting means includes means for pulse shapingat least one of said segments; a polarization rotator responsive to thepulsed radiation; and means for controlling the rotation of polarizationon said polarization rotation means to provide said segmentsubstantially in a square waveform.
 13. A system for pulse shapingpulsed radiation employed in photoexcitation comprising:a source ofpulsed radiation; means responsive to the pulsed radiation from saidsource for segmenting each pulse of radiation into temporally distinctsegments; means for separating the segments onto separate paths; andmeans responsive to the pulse segments on at least one of the differentpaths for producing photoexcitation; at least one further source ofpulsed radiation with the radiation applied to said photoexcitationproducing means; and wherein said segmenting means provides a segmentfor application to said photoexcitation producing means with an enhancedrise time leading edge occurring a fraction of the pulse durationsubsequent to the leading edge of the radiation pulse of said at leastone further source.
 14. The system of claim 13 further including meansfor controlling the rotation of polarization on said polarizationrotation means to provide said segment substantially in a squarewaveform.
 15. A system for pulse shaping pulsed radiation employed inphotoexcitation comprising:a source of pulsed radiation; meansresponsive to the pulsed radiation from said source for segmenting eachpulse of radiation into temporally distinct segments; means forseparating the segments onto separate paths; and means responsive to thepulse segments on at least one of the different paths for producingphotoexcitation; said segmenting means includes means for segmentingeach pulse to provide an enhanced rise time in at least one segment andin which said one segment is applied to the path for producingphotoexcitation; a polarization rotator responsive to the pulsedradiation; means for controlling the rotation of polarization on saidpolarization rotation means to provide said segment substantially in asquare waveform.
 16. A system for pulse shaping pulsed radiationemployed in photoexcitation comprising:a source of pulsed radiation;means responsive to the pulsed radiation from said source for segmentingeach pulse of radiation into temporally distinct segments; means forseparating the segments onto separage paths; and means responsive to thepulse segments on at least one of the different paths for producingphotoexcitation; said segmenting means include means for segmenting eachpulse to provide an enhanced rise time in at least one segment and inwhich said one segment is applied to the path for producingphotoexcitation. at least one further source of pulsed radiation withthe radiation thereof applied to said photoexcitation producing means;and wherein said segmenting means provides a segment for application tosaid photoexcitation producing means with an enhanced rise time leadingedge occurring a fraction of the pulse duration subsequent to theleading edge of the radiation pulse of said at least one further source.17. The system of claim 16 further including means for causing at leastone segmented radiation pulse to be delayed in application to thephotoexcitation producing means whereby its leading edge occurs thereinduring the other of the plural pulses.
 18. A system for pulse shapingpulsed radiation employed in photoexcitation comprising:a source ofpulsed radiation; means responsive to the pulsed radiation from saidsource for segmenting each pulse of radiation into temporally distinctsegments; means for separating the segments onto separate paths; andmeans responsive to the pulse segments on at least one of the differentpaths for producing photoexcitation; a plurality of the segmenting andseparating system wherein each respective radiation source provides adifferent frequency of radiation; and plural means each responsive tothe corresponding segments of plused radiation of different colors forsuperimposing one upon the other for application to said photoexcitationproducing means.
 19. A system for pulse shaping pulsed radiationemployed in phtoexcitation comprising:means defining an environment ofparticles having a characteristic time for being photoexcited; a sourceof pulsed radiation; the pulse duration of which is substantially longerthan said characteristic time; means responsive to the pulsed radiationfrom said source for segmenting each pulse of radiation into temporallydistinct segments, each shorter than the pulse duration provided by saidsource; means for separating the segments onto separate paths; and meansresponsive to the pulse segments on at least one of the different pathsfor producing photoexcitation of particles of said environment.